Magnetron and radio frequency circuitry therefor



Sept. 20, 1966 D. PETERSON 3,274,433

MAGNETRON AND RADIO FREQUENCY CIRCUITRY THEREFOR 2 Sheets-Sheet 1 Original Filed May 14, 1963 INVENTOR. DALE L. PETERSON MJX Ei ATTORNEY Sept. 20, 1966 D. L. PETERSQN MAGNETRON AND RADIO FREQUENCY CIRCUITRY THEREFOR Original Filed May 14. 1963 2 Sheets-Sheet 2 INVENTOR. DALE L. PETERSON wf afi ATTORNEY United States Patent Ofiice 3,274,433 Patented Sept. 20, 1966 3,274,433 MAGNETRON AND RADIO FREQUENCY CHRCUITRY THEREFOR Dale L. Peterson, Santa Clara, Calif., assignor, by mesn assignments, to Varian Associates, a corporation of California Continuation of application Ser. No. 280,324, May 14, 1963.. This application July 22, 1965, Ser. No. 480,228 4 Claims. (Cl. 315-3953) This application is a continuation of my co pending application Serial No. 280,324 filed May 14, 1963, now abandoned.

This invention relates to magnetrons and more particularly to three terminal magnetrons having associated R.F. circuitry.

A voltage tunable magnetron is a microwave oscillator tube the operating frequency of which can be varied over wide limits by means of a control voltage. These oscillators utilize a broadband periodic circuit which is held in close proximity to an electron stream and may be characterized as being radial transmission lines. The tubes have generally been supplied with exterior R.F. or microwave circuits. The performance of a voltage tunable magnetron is intimately associated with the characteristics of its external R.F. circuit. For example, the frequency range, power output, oscillation stability, spurious outputs etc. of a voltage tuned magnetron are determined to a great extent by its external RE. circuit.

Heretofore in the prior art, pillbox tuners and ridged waveguides have been used as external R.F. circuits for voltage tunable magnetrons. Such devices do not allow operation of the tunable magnetron at low frequencies due to their necessarily large size and bulk at such frequencies. Attempts have been made to present a uniform circumferential impedance to the voltage tunable magnetron with these devices by connecting a plurality of radially spaced coaxial lines to the tube and then joining them together outside the RF. circuitry by way of an impedance transformer to a single coaxial output line. The results are far from satisfactory and difficult problems are encountered when combining the power in each of the separate lines into a single output line. Also, such prior art devices do not permit simple and easy connection of a coaxial line to the output power of the voltage tunable magnetron.

Accordingly, an object of this invention is to provide improved R.F. circuitry for a voltage tunable magnetron.

Another object of this invention is to provide a voltage tunable magnetron having an RF. structure which operates efficiently at low frequencies.

Another object of this invention is to provide a substantially uniform circumferential impedance for a voltage tun-able magnetron having an RF. circuit.

Still another object of this invention is to provide a simple coaxial connection to a voltage tunable magnetron having an external R.=F. circuit.

A further object of this invention is to provide improved cooling of a voltage tunable magnetron having an external R.F. circuit.

A further object of this invention is to increase the efficiency of a voltage tunable magnetron having an external R.'F. circuit.

A still further object of this invention is to provide an improved external R.F. circuit for a voltage tunable magnetron which is an integral part of the evacuated portion of the magnetron.

These and other objects and advantages of the present invention are accomplished by uniquely combining a novel resonant cavity with a three terminal voltage tunable magnetron. The magnetron may include an interdigital line having a plurality of segments disposed in a circular array with at least three terminal rings disposed around the segments. Some of the segments are connected to one of the terminal rings which is electrically insulated from and located between the other terminal rings. A resonant cavity surrounds the magnetron and is conductively coupled to the other terminal rings thereby causing the electrically isolated terminal ring to be en closed by the cavity. At least one conductive rod is connected between the isolated terminal ring and a portion of the cavity remote from the terminal rings. Output means are associated with the cavity for extracting RF. power. The Q and operating frequency of the resonant cavity can be varied by changing the size and number of the conductive rods and the capacitance of the structure may be reduced by making the cavity an integral part of the evacuated portion of the magnetron.

This invention, as well as other features, objects and advantages thereof, will be readily apparent from consideration of the following detailed description relating to the annexed drawings in which:

FIGURE 1 illustrates in cross-section a voltage tunable magnetron having an RF. circuit in accordance with one embodiment of this invention;

FIGURE 2 is a perspective view of an interdigital line which may be utilized by the magnetron of FIGURE 1;

FIGURE 3 is a schematic cross-section illustration of a resonant cavity and three output terminals of a magnetron in accordance with this invention; and

FIGURE 4 illustrates in partial cross-section a modification of the embodiment shown in FIGURE 1.

Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in FIGURE 1 (which illustrates one embodiment of this invention) a three terminal voltage tunable magnetron '11 which is uniquely combined with a novel resonant cavity 12 that surrounds the magnetron. The magnetron includes a cathode support wall 13 at one end, a cold non-emitting cathode or electrode support wall 14 at the other end, and between the two a cylindrical interaction space 15 formed by an interdigital line. The interdigital line is formed by a plur-ality of thin, elongated segments 16 disposed in a circular array. A hot electron-emitting cathode 18 emits electrons which are guided into the interaction space 15 by a control electrode r19. A magnetic field (not shown) axially aligned with the interaction space 15 causes the electrons to spin around a cold non-emitting cathode or electrode 17 which is at the same potential as the hot cathode 18. The electrons thus travel in an essentially helical path axially along the tube until they are collected by the segments 16 of the interdigital line. The electrons travel this spirally-helical path because the control electrode 19 is at a positive potential relative to the hot cathode thereby attracting the electrons radially outward, because the electrons are effected by a force which is perpendicular to the axially aligned magnetic field, and because they are attracted into the interaction space 15 by a positive potential on the segments .16, which potential is more positive than the potential of the control electrode 19.

The cathode 18 is preferably a porous sintered-nickel body with barium, strontium, or calcium oxides interposed within the pores and is known as a matrix type cathode. Other types of cathodes, such as a hot emitting filament or a coated oxide cathode, may be used instead of a matrix type cathode. The preferred cathode 18 is mounted on a tubular sleeve 20. Between the cathode 18 and the tubular sleeve 20 is disposed a heating filament 21. The heating filament is preferably a double helically wound type which has its leads connected to a terminal ring 22 and a support or metallic vacuum wall 13, which are insulated from each other by a ceramic insulator ring 24. A ceramic backing ring 25 backs up the metalceramic seal at the terminal ring .22, and a ceramic backing ring 26 backs up the metal-ceramic seal at the end wall 13. The terminal ring 22 is also a sealing ring which has a flanged portion ;27 turned parallel to the axis of the tube. The axial sleeve 20, on which the hot cathode 18 is mounted, is preferably of thin-wall construction and acts as a heat dam on one side of the cathode holding the heat of the filament in the region of the cathode 18. The thin-wall axial sleeve is mounted at one end on a thin-wall conical support 28 which acts as a heat dam on the other side of the cathode. An aperture 29 is provided in the support 28 for easy evacuation. The conical support 28 in turn is mounted on the metallic end waill1'3. The other end of the axial sleeve 20 is received by the hollow cold non-emitting cathode 17.

The control electrode 19 may be made cylindrical with an inner diameter equal to the outer diameter of the interaction space formed by the segments 16. The control electrode 19 has a terminal ring 30 with a ceramic insulator 31 separating the ring from a metal sealing ring 32. A ceramic backing member 33 is disposed on the opposite side of the sealing ring 32. The segments 16 are mounted so that they form an interdigital line which has three terminal rings 34, 35 and 36. The terminal ring 30 and the ring terminals 34, 35 and 36 are separated by ceramic insulators 37, 38 and 39, respectively. An insulating member 14 supports the nonemitting cathode 17. An exhaust tubulation (not shown) is disposed in the cathode support area. The tube envelope is evacuated through the tubulation and then the end of the tubulation is nipped-off in a well known manner.

The segments 16 of the interdigital line are connected to the three ring terminals 34, 35 and 36. Reference to FIGURE 2, which illustrates a perspective view of the segments and ring terminals, shows that the interdigital line may contain eight segments a, b, c, d, e, f, and g, but more or less than eight segments may comprise an interdigital line. Every other segment, such as segments a, c, e, and g, is connected at a point between its ends to the ring 34 that is disposed between rings 35 and 36. The remaining segments, segments b, d, f, and h, have both of their ends connected to rings 35 and 36 as shown. The rings 35 and 36 are at radio frequency ground potential and act as ground planes or shields for electrically isolating the center or hot ring 34, which is R.F. isolated from the grounded rings 35 and 36.

It is to be understood that this invention is not limited to the voltage tunale magnetron described hereinabove in connection with FIGURE 1, for any three terminal magnetron device having a center terminal electrically isolated or insulated from the other terminals may be utilized to practice the present invention.

Surrounding the voltage tunable magnetron 11 is an annular shaped hollow resonant cavity 12 having an open inner circumference which forms two inner diameter surfaces 40 and 41, each of which are electrically connected or coupled to opposite ones of the grounded terminal rings, such as rings 35 and 36, respectively. The cavity may be fabricated from any suitable metal, such as oxygen-free high-conductivity copper. For purposes of illustration, a double re-entry cavity is shown in FIG- URE 1. However, as is more fully discussed hereinbelow, the present invention is not limited to such a resonant cavity for various resonant cavities may be utilized without departing from the spirit and scope of this invention.

Atleast one conductive rod 42 has one of its ends electrically connected to the center terminal ring 34 and its opposite end connected to a portion of the resonant cavity 12 remote from the terminal rings 34, 35 and 36. The rod 42 may be formed from any suitable metal, such as oxygen-free high-conductivity copper. Also, the rod 42 may comprise a non-conductive material having an exterior coating of conductive material thereon. Further, the rod 42 may take any desired geometric shape, al though in a preferred embodiment of the present invention a solid cylindrical rod was utilized as illustrated in FIGURE 1. As will be discussed hereinbelow in detail, a plurality of radially spaced rods may be utilized. The conductive rod 42 provides a DC. connection from the ring 34 to the other rings 35 and 36. However, due to the impedance of the rod 42 at R.F. frequencies the ring 34 remains lRfF. isolated from the rings 35 and 36.

RF. output means are associated with the resonant cavity and include an inner conductor 43, substantially similar to the rod 42, having one of its ends electrically connected to the center terminal ring 34 and its other end extending through an insulated from a portion of the cavity 12 remote from said terminal rings 34, 35 and 36. An outer conductor 44 is connected to the exterior of the resonant cavity and surrounds the inner conductor 43. A dielectric spacer separates the inner conductor 43 and the outer conductor 44 to form a simple output coaxial connection for extracting RF. power from the tube 1 1 and the cavity 12. If desired, the output coaxial means may form a vacuum tight seal with the cavity.

The apparatus illustrated in FIGURE 1 operates in the following manner: An external magnetic circuit (not shown) forms a relatively uniform magnetic field within the interaction space 15 which field is oriented parallel to the tube axis. The hot cathode 1'8 emits electrons which are attracted by the control electrode 19. The electrons, being thus subjected to a force which is oriented radially outward and perpendicular to the magnetic field, are caused to spin around the axis of the magnetron. The segments 16 around the interaction space 15, being more positive than the control electrode,

attract the spinning electrons within the interaction space 15. When the electrons are spinning within the interaction space, R.F. electromagnetic oscillations are formed in the segments 16 in accordance with known theories. The electrons spin in a somewhat helical path along the axis until they are collected by the segments 16 of the interdigital line.

The R.'F. oscillations in the segments cause an RF. potential difference to exist between the center terminal ring 34 and the other terminal rings 35 and 36 which are at RF. ground potential. This potential difference causes current to flow in the resonant cavity walls and in the rod 42 in a manner illustrated in FIGURE 3 which shows current flow at an instant in time. Referring now to FIGURE 3, there is illustrated a current path which begins at the two outer terminal rings 35 and 36, :fiows outwardly along the walls of the resonant cavity 1 2, through the rod-s 42 and 42 which are substantially similar to the rod 42 of FIGURE 1, and to the terminal ring 3 4. This current causes resonant oscillations in the resonant cavity 12. The RJF. power is extracted from the combined resonant cavity and magnetron by the coaxial output means illustrated in FIGURE 1 and described herein-above.

Referring again to FIGURE 1 wherein at least one rod 42 is illustrated, it is to be understood that a plurality of such rods radially spaced apart in the cavity 12 may be used and that they may take any desired geometric shape. In practicing this invention, three to six such rods have been utilized. By changing the size (crosssectional area) and number of the rods 42, the resonant cavity 12 may be tuned inasmuch as the number and size of the rods 42 used determines the Q and resonant frequency of the cavity 12. For example, by increasing the size and/or number of rods 42 utilized, the inductance of the cavity '12 is decreased and its resonant frequency is increased. Increasing the number of rods 42 increases the Q of the cavity by decreasing the coupling of the.

cavity to the load as now a smaller fraction of the cavity current flows in the center conductor of the coaxial line coupled through the cavity to one set of the interdigital fingers. Thus, less loading of the cavity is obtained and its Q increased. Accordingly, the apparatus as illustrated in FIGURE 1 can operate at low frequencies without the resonant cavity 12 being large and bulky because the cavity is capable of being tuned by a number of the rods 42. if desired, only a single rod 42 can be used. In this case, RF. power can be coupled out of the cavity by an R.F. coupling loop (not shown). It will be apparent to those skilled in the art that the use of a plurality of rods 42 Will cause the resonant cavity to present a uniform circumferential impedance to the magnetron 11. Also, the inner conductor 43 of the output coupling means may be tapered or otherwise shaped to obtain an impedance match between an output coaxial line and the resonant cavity.

Although a double re-entry cavity is illustrated in FIG- UR E 1, any type resonant cavity may be utilized in accordance with the principles of this invention. A double re-entry cavity is illustrated because of its low capacitance due to the relatively wide space between the portions 46 and 47 of the cavity. Because of the rods 42, the crosssectional area of the resonant cavity is determined only in part by the operating frequency range required.

If little or no frequency range is desired, a high Q resonant cavity may be used. However, if a wide frequency range is desired, a low Q resonant cavity is utilized. It will be apparent that the apparatus illustrated in FIGURE 1 has the advantage of allowing a simple coaxial connection for extracting RE. energy from the combined magnetron and resonant cavity.

The large surface area of the resonant cavity 12, which is connected to the terminal rings 34, 3 5 and 35, can be thermally tied to a magnet (-not shown) which produces an axial magnetic held, to efiiciently cool the cavity and interdigital line of the magnetron. Because the cavity presents a complete current path, it reduces RTE. leakage because the RF. energy is completely contained within the cavity 12.

Also, the insulating dielectric rings 38 and 3 9 which cause dielectric losses and which increase the capacitance present in the structure by which useful RF. power is shunted to ground can be removed as is illustrated in FIGURE 4. Referring now to FIGURE 4, there is illusstrated a magnetron 11 combined with a resonant cavity 1 2 in a manner similar to that illustrated in FIGURE 1. However, in the device of FIGURE 4, the insulating rings 38 and 39 of "FIGURE 1 have been removed to decrease dielectric losses and to decrease the capacitance of the apparatus. This structure requires that the resonant cavity 12 become an integral part of the evacuated portion of the magnetron llll. Accordingly, the resonant cavity 12 may be an integral part of the rings 35 and 35 or the cavity can be brazed to the terminal rings 35 and 36' to obtain a vacuum tight seal. Also, the inner conductor 43 may be an integral part of the ring 34 or may be merely connected thereto. The resonant cavity 12 is then evacuated at the same time the magnetron 11 is evacuated by means of a tubulation (not shown) in a well known manner.

Efiiciences up to 70% have been obtained and efiiciencies up to 85% can be reasonably expected by uniquely combining a magnetron and novel resonant cavity, as described hereinabove.

What has been described is a three terminal voltage tunable magnetron which is uniquely combined with a novel resonant cavity having conductive rods connected between a magnetron terminal ring which is electrically isolated from and located between the other magnetron terminal rings and a portion of the resonant cavity remote from the terminal rings. This improved structure permits operation at low frequencies, presents a substantial uniform circumferential impedance to the magnetron, provides a simple coaxial connection for extracting RF. power, improves cooling of the cavity and magnetron, improves the efliciency of the magnetron, and permits operation over a wide range of frequencies.

It should be understood, of course, that the above detailed description relates only to preferred embodiments of the present invention, and that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purpose of the disclosure which does not depart from the spirit and scope of the invention as set forth in the following claims.

What is claimed is:

1. A radio frequency circuit for an electron discharge device, said electron discharge device having at least three radio frequency terminals one of which is electrically isolated from the other terminals of said discharge device, comprising: a resonant cavity adapted to surround said electron discharge device and being coupled to said other terminals, and at least one elongated conductive rod the size of which can influence the Q and resonant frequency of said cavity coupled to and radially extending in its elongated direction between said electrically isolated terminal and a portion of said surrounding resonant cavity.

2. An electron discharge device comprising an interdigital line including a plurality of segments disposed in an array, some of said segments being electrically connected to each other and electrically insulated from the remaining segments, at least some of said remaining segments being electrically connected to each other, a resonant cavity surrounding said segments and being conductively coupled to said remaining segments, at least one elongated conductive rod conductively coupled to and radially extending in its elongated direction between said electrically insulated segments and a portion of said surrounding cavity, and output means associated with said cavity.

3. A magnetron comprising: an interdigital line including a plurality of segments disposed in a circular array, at least three terminal rings disposed around the segments, some of said segments being connected to one of said terminal rings which is radio frequency isolated from and located between the other terminal rings and the remaining segments being connected to said other terminal rings, a resonant cavity coupled to said other terminal rings causing said radio frequency isolated ring to be enclosed by said cavity, a plurality of elongated circumferentially spaced apart conductive rods each radially extending in its elongated direction and connected between said radio frequency insulated terminal and a portion of said surrounding cavity, and output means circumferentially spaced from said aforementioned plurality of rods and including an elongated inner conductor having one of its ends connected to said radio frequency isolated terminal ring and its other end extending through and insulated from said cavity, and an outer conductor connected to said cavity and surrounding said inner conductor.

4. A voltage tunable magnetron comprising an interaction space including an interdigital line having a plurality of segments disposed in a circular array, some of said segments being electrically connected to one another and radio frequency insulated from and located between the remaining segments, a hollow resonant cavity having an open inner circumference surrounding said plurality of segments, said remaining segments having their end portions electrically coupled to opposite sides of said opening thereby causing said radio frequency isolated segments to be enclosed by said cavity, a plurality of circumferentially spaced elongated conductive rods each radially extending in its elongated direction and electrically coupled between said radio frequency insulated segments and a portion of said surrounding cavity remote from said plurality of segments, and output means circumferentially spaced from said plurality of rods and including an electr-ical conductor having one of its ends connected to said 7 8 radio frequency isolated segments and its other end ex- 2,816,248 12/ 19 57 Pease 315-39.5=3 X tending through and insulated from said cavity. ,280 4/ 1 963 McLaughlin 315- 3973 3,158,780 11/1964 Gerlack 3l539.6 3 References Cited by the Examiner UNITED STATES PATENTS 5 HER-MA-N KARL SAALBACH, Primary Examiner. 2,463,416 3/ 1949 Nordsieck 315-3 913 X E. LIEB'ERMLAN, S. CHATMOIN, JR.,

2,607,905 7/1952 Ludi 31539.77 X Assistant Examiners. 

1. A RADIO FREQUENCY CIRCUIT FOR AN ELECTRON DISCHARGE DEVICE, SAID ELECTRON DISCHARGE DEVICE HAVING AT LEAST THREE RADIO FREQUENCY TERMINALS ONE OF WHICH IS ELECTRICALLY ISOLATED FROM THE OTHER TERMINALS OF SAID DISCHARGE DEVICE, COMPRISING: A RESONANT CAVITY ADAPTED TO SURROUND SAID ELECTRON DISCHARGE DEVICE AND BEING COUPLED TO SAID OTHER TERMINALS, AND AT LEAST ONE ELONGATED CONDUCTIVE ROD THE SIZE OF WHICH CAN INFLUENCE THE Q AND RESONANT FREQUENCY OF SAID CAVITY COUPLED TO AND RADIALLY EXTENDING IN ITS ELONGATED DIRECTION BETWEEN SAID ELECTRICALLY ISOLATED TERMINAL AND A PORTION OF SAID SURROUNDING RESONANT CAVITY. 